The comment // I2C0 (external) pins is misleading, because although configuring the bus with machine.I2C0.Configure(machine.I2CConfig{}) the external pins will not be used. The reason seems to be the same code for all "I2Cx" together with the defaulting to internal.

To get the external pins to work you need to set the pins explicitly like this: machine.I2C0.Configure(machine.I2CConfig{SCL: machine.SCL0_PIN, SDA: machine.SDA0_PIN}).